When analysing medicinal products with the aid of vibration spectrometry, one has resorted primarily to infrared (IR) spectrometry, since IR spectra provide detailed information on the molecular structure. Today's vibration spectrometry is divided into the following three main areas:
1. the middle part of the IR range; PA1 2. the upper part (NIR) of the IR range; and PA1 3. Raman spectrometry.
Obtaining and evaluating Raman spectra was previously, beginning in the year 1928 when the Raman effect was discovered, a fairly time-consuming and difficult procedure. The development of modern CCD units, advanced lasers and not least powerful hardware and software for data processing has, however, resulted in a considerable increase in the number of applications of Raman-based analysis. The advent of the FT technique (Fourier transform) for analysing signals obtained through Raman spectrometry constituted a breakthrough of particular importance. This technique enabled excitation in the NIR range (Near Infrared) resulting in a sufficiently powerful Raman signal without any interfering fluorescence. The FT-Raman technique is today used commercially for qualitative as well as quantitative analysis within a great many different fields, such as the analysis of polymers, hot gases (flames), medicinal products and biomaterial.
Being well-known to those skilled in the art, the Raman effect need not be described in more detail here. However, when an excitation beam (in practice a laser beam) is directed towards a sample (gaseous, liquid or solid) that is to be analysed, part of the incoming excitation beam is scattered in all directions at another wavelength (shorter or longer) from molecules whose polarization is altered when they are caused to vibrate by the field generated by the excitation beam. By intercepting the retransmitted radiation, one obtains a Raman spectrum upon which a quantitative as well as a qualitative chemical analysis can be based. When solid samples are measured, use is normally made of so-called back-scattering geometry, which means that the analysis involves the light that is reflected at an angle of 180.degree. to the direction of incidence of the excitation beam.
When Raman spectrometry is applied to solid samples, one obtains a fairly large depth of penetration into the sample, for instance in the order of 1 mm. Bearing this in mind, one realises the potential of the Raman technique for e.g. the analysis of the ingredients, especially the active substances, of whole tablets.
In the analysis of solid samples, Raman spectrometry does, however, suffer from an inconvenience not encountered in NIR spectrometry, namely that the excitation beam (in practice a laser beam), has to have a very narrow focus. As a result, it is only possible to analyse a relatively small part of the total volume of the sample. If the sample, for instance a tablet, is homogeneous, there are no problems. If, however, the sample is inhomogeneous, as is often the case for tablets, the results of the analysis are not representative of the whole tablet, for instance as regards the concentration of an active substance.
In an illustrative example of Raman analysis of a tablet, the "part volume" of the total volume of the sample actually analysed by the beam may be in the form of a cone converging in the direction of the excitation beam and having a height of approximately 1 mm and a base diameter of approximately 0.4 mm at the impingement surface on the sample. For a typical tablet having a diameter of 10 mm and a thickness of 4 mm, the conical part volume analysed thus constitutes but a small part of the total volume of the tablet. This problem is not encountered in NIR spectrometry, where the excitation beam can be made to cover the entire irradiated surface of the sample.
This invention has been developed in an effort to solve, or at least substantially reduce, the above-mentioned problems from which suffer the prior-art technique in Raman spectrometry involving solid samples.
By the expression "solid samples" is here meant samples in which the analysed substance in itself is a solid unit, such as a tablet, samples where the substance to be analysed consists of a more or less compacted powder, or some other solid object placed in a sample container.